1. Field of the Invention
The invention relates to a method of surface treating and compressively prestressing by peening the wall of at least one annular recess approximately in the shape of a dovetail, formed at the periphery of a rotor rim, said annular recess opening to the outside via an annular lateral mouth and having at least two recess ends opening into, respectively, at least one blade introduction opening.
In particular, it is necessary to compressively prestress the wall of an annular recess for hammered attachment of the blades to a turbomachine rotor capable of containing blade roots, the lines of contact between the annular recess and each blade root are highly stressed. What happens in operation is that the turbine or fan blades immobilized in an annular recess by various means, are subjected to considerable centrifugal forces leading to significant frictional wear at these lines of contact. This frictional wear reduces the life of the parts in operation and leads to them having to be changed regularly.
2. Summary of the Prior Art
In order to improve the fatigue strength of the turbine rotor and to harden the surface of the annular recesses in the region of the lines of contact, it is known practice for the annular recesses to be peened using beads ejected from a compressed air nozzle introduced into each recess. The beads cause permanent compression of the treated surface over a shallow thickness so as to oppose the onset and propagation of cracks at the surface of the part.
These nozzles are able to project only microbeads of a diameter smaller than 1 mm, and typically of between 0.3 mm and 0.5 mm. Furthermore, the distribution of the beads, both in terms of position and in terms of speed, follows a Gaussian distribution.
This method is necessarily lengthy because the entire surface area of the cavity is treated only by a succession of local treatments which may, furthermore, introduce unwanted local deformation and cause the incrustation of bead residue.
In addition, the peening performed is light in order not to generate excessive roughness in the region of the treated zone, and this leads to a limited hardening of the surface. The problem is that since the diameter of the projected beads is small, the more intense the peening, the greater the damage caused to the surface finish.
Finally, the method, which involves numerous parameters, is difficult to control and difficult to reproduce.
The object of the invention is to propose a method for peening the annular recesses of turbine rotors, which makes it possible to create reproducible prestresses in all the recesses, in a relatively short period of time, while at the same time allowing relatively intense peening, that is to say a greater depth of compression at the surface. This is to be achieved without the introduction of deformation, while at the same time limiting the damage caused to the treated surface.
According to the invention, there is provided a method of surface treating and compressively prestressing by peening a wall of at least one annular recess approximately in the shape of a dovetail, formed in the periphery of a rotor rim, said annular recess opening to the outside via an annular lateral mouth and having at least two recess ends opening into, respectively, at least one blade introduction opening, said method comprising the steps of:
a) providing a sonotrode arranged within a sleeve, said sonotrode having a vibratory surface and ultrasonic means for vibrating said vibratory surface, and said sleeve having opposed closing-off means capable of sliding in the recess;
b) placing a plurality of beads of a predetermined diameter on said vibratory surface of said sonotrode with said sonotrode arranged in an intermediate position in which said sleeve surrounds said vibratory surface;
c) placing said introduction opening facing said sonotrode;
d) moving said sonotrode together with said sleeve substantially vertically toward said blade introduction opening to place them in a peening position in which said closing-off means face said recess ends that open into said opening;
e) turning said rotor rim about an axis of rotation arranged horizontally in such a way as to bring said closing-off means into a first recess end so as to form a closed moving chamber containing said beads; and
f) operating said ultrasonic means to vibrate said vibratory surface of said sonotrode whereby said beads are mobilized in said moving chamber, and, by uniformly turning said rotor rim about said axis of rotation until said second recess is reached, ultrasonically peening the entirety of said wall of said recess.
The method makes it possible to obtain a uniform distribution of the beads within the chamber. As the beads travel in random directions, they strike the walls of the cavities at varying angles, and this improves the surface finish by comparison with beads projected by a nozzle in one predominant direction.
The method requires only a small quantity of beads. It is therefore possible to use beads which are of high quality in terms of hardness and in terms of geometry, for example steel or ceramic ballbearings, which allows the surface finish to be better preserved.
According to a preferred embodiment of the invention, the method further includes the step of moving said sonotrode substantially vertically so that said vibratory surface is level with said lateral mouth of said recess, after the step of moving said sonotrode and said sleeve together into said peening position.
As the volume in which the plurality of beads is distributed is smaller than the volume obtained when the vibratory surface remains outside the mouth, the method requires a shorter peening treatment time.
According to another preferred embodiment, the peening step is conducted at least three successive times on said wall by reversing the direction of rotation of said rotor rim each time one of said recess ends is neared.
Not all of the wall of the annular recess is subjected to the impact of the beads at the same time, and this leads to risks that the cavity, particularly the recess containing the blade root, will become deformed, given the magnitude of the diameter of the rim with respect to its thickness. The fact that the wall is treated in three successive treatments allows rim deformation to be compensated for considerably.
Of course, it is also possible to carry out three successive peening operations keeping the same direction of rotation of the rotor rim and halting peening when the closing-off cheeks and the vibratory surface near a first mouth end, then reactivating the plurality of beads when the closing-off cheeks and the vibratory surface are out of the introduction opening again introduced into the mouth.
According to yet another preferred embodiment, the method includes the step of placing a deflector in said moving chamber.
The deflector may be of approximately triangular geometry with sides parallel to the divergent wall portions so as to reduce the effect of the peening of the zone situated between said deflector and the bottom of the cavity.
According to an even yet further preferred embodiment, the method further includes the steps of rotating said rotor part about said axis of rotation so as to bring said closing-off means into said blade introduction opening after peening said first recess;
moving said sonotrode and said sleeve substantially vertically toward a lowered position;
moving said sleeve and said sonotrode together substantially horizontally toward an introduction opening of a second annular recess;
bringing said opening up to face said sonotrode; and
positioning said sonotrode, supporting said beads, and said sleeve in said raised peening position to peen said second annular recess.
At the end of the treatment, the sonotrode is advantageously moved approximately vertically toward a lowered position in the sleeve, in which position the plurality of beads can be driven from the vibratory surface toward a reservoir through slots formed in the sleeve, after the annular recesses formed on the rotor rim have been peened.
Thus, the beads can easily be recovered so that they can either be reused in a subsequent treatment or be replaced.
According to another yet further preferred embodiment, the predetermined diameter of said beads exceeds 0.8 mm.
The beads used in the method according to the invention have a larger diameter than the diameter of the beads that can be projected by a nozzle, which means that the peening can be more intense while at the same time causing less substantial damage to the surface.
The invention also provides an apparatus for implementing the method, said apparatus comprising a sonotrode mounted in a sleeve equipped with closing-off means, said sonotrode being capable of projecting beads of a predetermined diameter into a moving chamber delimited by a wall portion of said annular recess,
said vibratory surface of said sonotrode arranged near said mouth of recess, and said closing off means;
means for producing ultrasonic oscillations capable of exciting said sonotrode;
first means for moving said sonotrode substantially vertically; and
second means for moving said closing-off means substantially vertically;
a first clearance, smaller than said diameter of said beads, being formed between said sonotrode and said sleeve; and
a second clearance, smaller than said diameter of said beads, being formed between said closing-off means arranged in said annular recess and said annular recess.
The apparatus may comprise a number of acoustic assemblies each comprising a sonotrode and a sleeve, arranged around the rotor rim, said acoustic assemblies being capable of moving in an axial direction of the rotor rim.
According to a preferred embodiment, there is provided a third clearance, smaller than said diameter of said beads, formed between the mouth and said vibratory surface arranged in said mouth.
According to a further preferred embodiment, there is provided a fourth clearance, smaller than said diameter of said beads, formed between an end of said sleeve and said mouth which, when the vibratory surface is not closing off the mouth, ensures the chamber is sealed correctly.
The various elements of the apparatus are arranged in such a way that no bead can jam the elements that are capable of moving. Furthermore, the sonotrode and the sleeve comprising the closing-off means have a geometry tailored to the shape of the cavity to be treated. In particular, the vibratory surface is of a shape that complements the mouth, and the closing-off means are formed in such a way as to seal the chamber correctly.
According to a yet further preferred embodiment, said second means for moving said closing-off means are capable of moving said closing-off means and said sonotrode at the same time.
According to an even yet further preferred embodiment, said first means for moving said sonotrode and said second means for moving said closing-off means can advantageously be operated simultaneously.
At the beginning of the treatment, the sleeve and the sonotrode are placed in an intermediate position in which the space generated by the closing-off means of said sleeve and the vibratory surface of said sonotrode constitutes a reservoir for the beads. Next, said sleeve and said sonotrode supporting the beads are moved jointly with the same movement. Finally, the sonotrode and/or the closing-off means can be moved individually, depending on the geometry of the cavity to be treated, until, respectively, the vibratory surface closes off the mouth of the cavity and the closing-off means close off the openings of the cavity.
According to yet another preferred embodiment, said apparatus further comprises support means for supporting said rotor rim that is to be treated and means for driving said support means to cause said annular recess to rotate uniformly.
The apparatus advantageously comprises means for driving the beads from the vibratory surface toward a reservoir. These simple means make it possible to drive the beads toward the reservoir during a treatment if necessary, or at the end thereof, either to refresh the beads or to replace them.
Advantageously, the apparatus further comprises means for moving said sonotrode and said sleeve substantially horizontally.
A number of annular recesses, arranged on a rotor rim and having the same cross-section may thus be treated with one and the same apparatus which is movable in such a way as to position the sonotrode and the sleeve to face the recess in question.